Since the inception of the Clean Water Act of 1974, municipal and industrial wastewater treatment facilities have had to dispose of solids separated from water during treatment. The aqueous solution of these solids is often termed sludge; however, for clarity, the term aqueous solids is used herein where said aqueous solids are aqueous primary solids and/or aqueous secondary solids. Aqueous primary solids are defined as aqueous solids that are separated from the treated water in primary treatment; primary treatment physically separates solids from the treated water, usually in a clarifier or a dissolved air flotation device and often with a chemical coagulant. Aqueous primary solids can contain organic and/or inorganic solids. Aqueous secondary solids, bio-solids, are defined as aqueous solids that are separated from the treated water in secondary treatment; secondary treatment is the biological treatment in a water treatment plant, usually a wastewater treatment plant. Aqueous secondary solids nearly always contain organic solids and may contain inorganic solids. Aqueous solids, be it aqueous primary solids and/or aqueous bio-solids, are normally sent to digestion. In digestion, the solids volume within the aqueous phase is reduced by bacteria that consume, digest, the separated aqueous solids. The performance of digestion is determined by the reduction of Volatiles in the aqueous solids. Volatiles are defined in the laboratory, as the solids remaining on a filter from a filtered sample after the filtered sample is heated to approximately 102° C., yet do not remain after a second heating to approximately 600° C. This mass measurement difference is a definition of the heavier organic content of the filter sample and is therefore an estimation of the biological content and/or organic biological food content of the solids in an aqueous sample. In a mesophilic digestion system, the percent Volatiles reduction is normally 40 to 50 percent. In thermophilic digestion, the percent Volatiles reduction is normally greater than mesophilic and can be as high as 55 to 65 percent. Mesophiles are defined as bacteria that operate between the temperatures of approximately 40 and 105° F. Thermophiles are defined as bacteria that operate between the temperatures of approximately 115 and 165° F. To manage transportation and disposal costs, nearly all wastewater treatment facilities prefer to reduce the Volatiles content of the digested solids as much as is economically practical.
After digestion, the final digested solids product (Digested Solids) must be properly disposed. Disposal of the Digested Solids (DS) is normally accomplished by either land application or by disposal in a landfill. To minimize the handling and transportation expense of the DS, the water content of the DS is normally reduced from approximately 94-97 percent, in digestion, to approximately 75 percent by chemical treatment utilizing a cationic polyacrylamide and mechanical separation utilizing a belt press, centrifuge, drying bed or other type device. To reduce the water content further, many facilities incorporate heated air-drying, evaporative air-drying, or a combination thereof. A drier product is required if the DS is stored for an extended time. DS placed in storage having a moisture content of greater than approximately 15 percent, yet less than approximately 90 percent, have the capability of spontaneous combustion. Many facilities use this biochemistry to heat treat the DS via composting, thereby to reduce the pathogen content of the DS.
Municipal wastewaters, and usually industrial wastewaters, generally contain four types of human pathogenic organisms: bacteria, viruses, protozoa and helminthes (parasitic worms). The actual species and density of pathogens contained in the raw wastewater will depend on the health of the particular community and/or the inclusion of significant rainwater runoff from animal sources. The level of pathogens contained in the untreated DS will depend on the flow scheme of the collection system and the type of wastewater treatment. For example, since pathogens are primarily associated with insoluble solids (non-volatile solids), untreated primary solids have a higher density of pathogens than the incoming wastewater.
Since pathogens only present a danger to humans and animals through physical contact, one important aspect in land application of DS is to minimize, if not eliminate, the potential for pathogen transport. Minimization of pathogen transport is accomplished through reduction of vector attraction. Vectors are any living organism capable of transmitting a pathogen from one organism to another either directly or indirectly by playing a key role in the life cycle of the pathogen. Vectors that are specifically related to DS could most likely include birds, rodents and insects. The majority of vector attraction substances contained in the DS are in the form of Volatiles. If left unstabilized, Volatiles will degrade, produce odor and attract pathogen-carrying Vectors.
On Feb. 19, 1993, the National Sewage Sludge Use and Disposal Regulations (EPA's Chapter 40 Code of Federal Regulations Part 503 and commonly referred to as the EPA's 40 CFR 503 Regulation) were published in the Federal Register. The US EPA's 40 CFR 503 regulation define DS treatment methods that transform DS into Class A DS; Class A DS is DS regarded substantially free of pathogens and Vector attraction. In essence, the Regulation establishes several categories in terms of stabilization, pathogenic content, beneficial reuse and disposal practices for all land-applied DS. These regulations set forth: chemical methods, temperature methods, methods that include a combination of chemical and temperature, as well as other methods including composting to treat DS for land application. Since 1993, experience has taught that the most reliable methods of Vector reduction are the temperature and/or chemical methods. The temperature methods include direct heating and thermophilic digestion.
However, the direct heating methods are rather expensive, as are the chemical methods. The chemical methods require raising the pH of the DS to a minimum of 12 utilizing an oxidizer; typically, lime is used. This is expensive, requiring nearly a 10 percent by mass dosage of lime; further, very alkaline DS is not a good fertilizer for land application. The temperature methods require heating the DS to a minimum of 50° C. for a specified period of time that is dependent on the amount of temperature above 50° C. This is expensive due to the energy requirement and the cost of heating a significant mass of aqueous solids.
The most economical method of disinfection involves Thermophilic Digestion (TD). TD is inexpensive from the standpoint of the cost of disinfection. Energy cost is minimal due to the thermophilic process itself. In the case of Aerobic TD (ATD), digestion naturally occurs so exothermically that once initiated, operating temperature is self-sustaining. In the case of Anaerobic TD (AnTD), the hydrocarbon gas produced in digestion can be sent to fire a boiler producing steam to maintain required temperature. Moreover, as mentioned previously, a side benefit to Vector reduction is an increase in the reduction of Volatiles by TD, thereby reducing the volume of aqueous solids. However, ATD and AnTD (TD in general) have significant issues in relation to odor and to dewatering cost. The dewatering cost of TD aqueous solids (TDS) is much more than that of Mesophilic Digested aqueous solids (MDS) due to the nature of thermophilic bacteria as compared to mesophilic bacteria. While messophilic bacteria naturally secrete tackifying polysaccharides to initiate floc formation, thereby naturally creating a microfloc, thermophiles do not. This biological difference can make the dewatering cost of TDS expensive and render the process of TD uneconomical. Previous work can be referenced in U.S. Pat. Nos. 5,846,435 and 5,906,750. However, these patents do not incorporate a means of controlling the odor of the TDS. TDS have strong ammonia and sulfide odors that are objectionable and that attract Vectors. Further work is documented in U.S. Pat. No. 6,083,404, wherein a three component system of dewatering TDS is presented; however, this patent has no method for controlling sulfide or ammonia.
Bacteria, Volatiles, contain a significant amount of protein and lipoic acids. A very large portion of the bacteria cell contains the proteins, DNA and RNA sequences, for cell reproduction. Significant portions of these proteins are amino acids. Lipoic acids contain sulfur. Amino acids, DNA and RNA contain nitrogen. As a result, the volatiles digestion releases ammonia and sulfide(s). While sulfidic odors are present in both MDS and in TDS, strong and objectionable ammonia odors have been specifically associated with TDS. The digestion process itself is an oxidation process of the volatiles. As such, the release of ammonia and sulfide(s) biologically occurs. However, it does appear that the mesophilic digestion (MD) process uses a much larger portion of the ammonia as a nutrient and/or in a conversion to nitrogen gas than does the TD process. Monitoring of ammonia nitrogen levels in TDS has found the ammonia to measure as high as 1500 to over 2000 ppm. At such levels, most of the ammonium hydroxide converts to ammonia gas. Ammonia gas is known to be toxic at these concentrations to all mesophilic organisms. Ammonia gas in the final DS can make land application of the DS objectionable; ammonia gas in the TD process can make the process itself objectionable. Further, since both the MD and the TD process create sulfide(s), the biological conversion of ammonia to nitrates, much less to nitrogen, is impractical. Sulfide(s), hydrogen sulfide and sulfur dioxide, are toxic to nitrifying bacteria. Having sulfide toxicity, MD cannot perform nitrification to remove ammonia odor from the DS, whether the DS be TDS or MDS. Further Nitrifiers, usually nitrosomonas and nitrobacter, are mesophilic bacteria, which cannot live above 105° F.; therefore, nitrification is impractical in TD.
Previous work in thermophilic digestion has been done by Ort, U.S. Pat. No. 4,040,953, which actually suggests that thermophilic digestion has certain advantages including a lower solids retention time and more readily dewatering characteristics. There is no discussion by Ort that TD would have odor issues, whether ammonia or sulfide(s). There is no discussion by Ort that TD would have dewatering issues. U.S. Pat. No. 5,492,624 presents the ATD, wherein there is no mention of dewatering or odor issues in relation to TD or in relation to the presented ATD. U.S. Pat. No. 6,203,701 presents an ATD process and apparatus; again, odor and dewatering issues are not discussed. Literature published in November of 2001 indicates a lack of understanding for both the source of thermophilic digestion odors and for a solution. “The Future of Solids Treatment?” Water, Environment and Technology, Vol. 12, No. 11, pp. 35-39. This literature documents that there are odor issues with thermophilic bio-solids; within this document there is no understanding of the source of the odor or any anticipated solution.
This instant invention identifies sulfide(s) as both an odor component and as a component to inhibit nitrification, thereby limiting nitrification. Previous work in U.S. Pat. No. 6,136,193 identifies thiobacillus and thiobacillus denitrificans as biological cultures that will remove sulfide(s) from sulfide laden aqueous systems. While this patent does recommend the use of thiobacillus with other biological cultures, this patent has no discussion of methods of TD and has no method of dewatering TDS.
Concentrations of sulfide(s) as low as 5 ppm are known to inhibit nitrification and to begin killing nitrosomonas. Concentrations of sulfide(s) as low as 3 ppm are inhibitory to nitrosomonas. As referenced in U.S. Pat. No. 5,705,072, the inhibitory and lethal aspects of sulfide(s) to nitrosomonas can be controlled by either oxidation of the sulfide(s) to sulfate or with the addition of a strain of bacteria to consume, thereby control, the sulfides. This can be performed with at least one of thiobacillus and/or thiobacillus denitrificans. However, this patent does not present TD, TDS or provide for dewatering. Further, there is no oxygen or available electron donor in the AnTD; therefore, a method is needed to control sulfide(s) and ammonia in the TDS from AnTD. Nitrosomonas and nitrobacter cannot live in an anaerobic or a thermophilic environment. Every pound of ammonia converted to nitrates by nitrification, requires approximately four pounds of oxygen, while thiobacillus and thiobacillus denitrificans are mesophiles; therefore, both the ATD and the AnTD operate at temperatures above the operating range of thiobacillus, thiobacillus denitrificans, nitrosomonas and nitrobacter.
Ammonia Nitrification
For treating the ammonia content of wastewaters, certain aerobic mesophilic autotrophic microorganisms can oxidize ammonia to nitrite, which can be further microbially oxidized to nitrate. Said reaction sequence is known as Nitrification. Nitrification reduces the total ammonia-nitrogen content of the wastewater. Ammonia is removed from the wastewater by bacterial oxidation of ammonia to nitrate (NO3), using bacteria that metabolize nitrogen. Nitrification is carried out by a limited number of bacterial species and under restricted conditions including a narrow range of pH and temperature and dissolved oxygen levels, along with reduced Chemical Oxygen Demand (COD) and reduced Biological Oxygen Demand (BOD) levels. Atmospheric oxygen is used as the oxidizing agent. Nitrifying bacteria grow slowly and nitrogen oxidation is energy poor in relation to mesophilic or thermophilic carbon oxidation. In addition, nitrification is inhibited by the presence of a large number of compounds, including organic ammonium compounds, sulfide(s) and the nitrite ion (NO2). Furthermore, nitrifying bacteria subsist only under aerobic conditions and require inorganic carbon (CO3− or HCO3−) for growth. The most common sequence is: 1
Approximately 4 pounds of oxygen and approximately 6 pounds of carbonate and/or bicarbonate are required for every pound of ammonia converted to nitrate.
It may be worthwhile to note that the term “ammonia” is used in the art to describe “ammonia as a contaminant in wastewater”. “Ammonia” refers, in this art, to the NH4+ ion that exists in aqueous solution and that is acted on microbially, with the following equilibrium existing in the aqueous solution:
As the total ammonia concentration approaches approximately 150 ppm and the pH approaches approximately 8.0, this reaction is driven to the right. At ammonia concentrations of approximately above 350 ppm and the pH approximately above 8.0, ammonia gas (NH3) toxicity exists in the water. In addition to being toxic, ammonia gas is volatile, having a significant vapor pressure.The Rancidity Process
The most practical recycle application of aqueous solids, whether MDS or TDS, is land application as a fertilizer. Further, most governmental agencies are requiring a Class A product for land application of DS. Upon application as a fertilizer, the aqueous solids, MDS or TDS, will decompose into fertilizer naturally. Bio-Solids are a natural organic fertilizer.
However, prior to land application, the aqueous solids can decompose by the rancidity process. The rancidity process is the natural degradation process for protein products. The rancidity process proceeds by the creation of sulfuric acid from sulfur in the lipoic acids. As the pH drops by the creation of sulfuric acid, the acidic pH further breaks apart amino acids and lipoic acids; this further creates sulfuric acid from the lipoic acids while releasing ammonia from the amino acids. While it appears sensible to oxidize sulfide odors of the aqueous solids or to control the rancidity process with an oxidizer, typical oxidation methods are not practical. Typical oxidizers such as caustic, potash, soda ash and lime, etc. are not self-buffering. These chemicals will increase the pH over 10 having the capability to increase the pH over 12. As the pH increases above 10, oxidation of the lipoic acids and of the amino acids releases sulfide(s) and ammonia. Due to this process, the addition of typical oxidizers will only accelerate the degradation of lipoic acids and amino acids. Further, most pH stabilizers are not self-buffering. Sodium hydroxide has a very strong ability to neutralize acids, but is not self-buffering. In the case of recycled proteins, stabilization of acid degradation reactions within 12 hours requires an amount of sodium hydroxide that will oxidize the proteins, thereby increasing degradation reactions. Unfortunately, pH stabilizers that have been determined to be self-buffering do not have a very strong ability to neutralize acids. Although sodium carbonate and sodium bicarbonate are self-buffering, they are limited in ability to neutralize acids.
U.S. Pat. No. 6,066,349 presents a method of controlling the rancidity process within proteins; however, this patent does not include a process for the control of odors from aqueous solids or TDS, or for the dewatering of TDS.
Vectors and the Environment
To control Vectors (a.k.a. pathogen transport) in our environment, the US EPA and many state agencies are requiring production of Class A bio-solids for land application of bio-solids. In combination with this legislative and regulatory trend, solid waste is becoming an issue to municipalities as landfill sites become more difficult to locate and permit. Since bio-solids are a natural and organic fertilizer, a process to economically produce Class A aqueous solids without an appreciable objectionable odor is needed. Further, a process that helps to insure that DS do not become attractive to Vectors would be beneficial to the environment, as well as to human and animal health.